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FreeBSD-EN-18:08.lazyfpu                                        Errata Notice
                                                          The FreeBSD Project

Topic:          LazyFPU remediation causes potential data corruption

Category:       core
Module:         kernel
Announced:      2018-09-12
Credits:        Gleb Kurtsou
Affects:        FreeBSD 10.4-STABLE, 11.1 and later.
Corrected:      2018-07-31 10:18:30 UTC (stable/11, 11.1-STABLE)
                2018-09-12 05:08:49 UTC (releng/11.2, 11.2-RELEASE-p3)
                2018-09-12 05:08:49 UTC (releng/11.1, 11.1-RELEASE-p14)
                2018-08-03 14:12:37 UTC (stable/10, 10.4-STABLE)

For general information regarding FreeBSD Errata Notices and Security
Advisories, including descriptions of the fields above, security
branches, and the following sections, please visit
<URL:https://security.FreeBSD.org/>.

Special Note: While SA-18:07.lazyfpu has been fixed in 10.4-STABLE, it has
yet to be released for 10.4-RELEASE.  As such, this EN does not apply for
that release.  Once SA-18:07.lazyfpu has been updated for 10.4-RELEASE,
this EN will be incorporated at that time.

I.   Background

The recent security advisory titled SA-18:07.lazyfpu resolved an issue in the
floating point unit (FPU) state handling.

II.  Problem Description

As a result of fixing the issue described in SA-18:07.lazyfpu, a regression
was introduced.  FPU state manipulation did not sufficiently prevent context
switches potentially allowing partially modified FPU context to be switched
out.  Upon returning the thread to a running state, stale FPU context could
be reloaded.

III. Impact

The regression could potentially cause an inconsistent FPU state, leading to
data corruption.

IV.  Workaround

No workaround is available.

V.   Solution

Perform one of the following:

1) Upgrade your system to a supported FreeBSD stable or release / security
branch (releng) dated after the correction date.

Afterward, reboot the system.

2) To update your system via a binary patch:

Systems running a RELEASE version of FreeBSD on the i386 or amd64
platforms can be updated via the freebsd-update(8) utility:

# freebsd-update fetch
# freebsd-update install

Afterward, reboot the system.

3) To update your system via a source code patch:

The following patches have been verified to apply to the applicable
FreeBSD release branches.

a) Download the relevant patch from the location below, and verify the
detached PGP signature using your PGP utility.

[FreeBSD 11.x]
# fetch https://security.FreeBSD.org/patches/EN-18:08/lazyfpu-11.patch
# fetch https://security.FreeBSD.org/patches/EN-18:08/lazyfpu-11.patch.asc
# gpg --verify lazyfpu-11.patch.asc

b) Apply the patch.  Execute the following commands as root:

# cd /usr/src
# patch < /path/to/patch

c) Recompile your kernel as described in
<URL:https://www.FreeBSD.org/handbook/kernelconfig.html> and reboot the
system.

VI.  Correction details

The following list contains the correction revision numbers for each
affected branch.

Branch/path                                                      Revision
- -------------------------------------------------------------------------
stable/10/                                                        r337254
stable/11/                                                        r336963
releng/11.1/                                                      r338607
releng/11.2/                                                      r338607
- -------------------------------------------------------------------------

To see which files were modified by a particular revision, run the
following command, replacing NNNNNN with the revision number, on a
machine with Subversion installed:

# svn diff -cNNNNNN --summarize svn://svn.freebsd.org/base

Or visit the following URL, replacing NNNNNN with the revision number:

<URL:https://svnweb.freebsd.org/base?view=revision&revision=NNNNNN>

VII. References

The security advisory that introduced the regression is available at
<URL:https://security.FreeBSD.org/advisories/FreeBSD-SA-18:07.lazyfpu.asc>

The latest revision of this advisory is available at
<URL:https://security.FreeBSD.org/advisories/FreeBSD-EN-18:08.lazyfpu.asc>
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